1. Field of the Invention
The invention relates to a tomosynthetic image reconstruction method, especially suitable for mammography, in which a tomosynthetic 3D x-ray image is assembled from individual digital images recorded from a number of different projection angles. In addition the invention relates to a diagnostic device operating with such a method.
2. Description of the Prior Art
Mammography involves x-ray examination of the female breast for the purpose of detecting tumors at the earliest possible stage. By constantly improving mammography methods an attempt is made to generate x-ray images supplying a high level of information, in order to distinguish beneficial from harmful changes and to reduce the number of incorrect findings, i.e. the number of suspicious findings that are caused by non-harmful changes, and the number of the undiscovered harmful tumors. In conventional x-ray mammography in this case a two-dimensional single image of the compressed breast is produced in a single projection direction. Since in such a projection the tissue layers lying behind each other in the direction of the x-ray beam are overlaid, heavily-absorbent beneficial structures can overlay a harmful tumor and render such detectability more difficult.
To avoid this, mammography methods known as tomosynthesis are known from Dobbins J T, III, Godfrey D J. “Digital x-ray tomosynthesis: current state of the art and clinical potential”, Physics in Medicine and Biology 48, R65-R106, 2003, in which individual images of the female breast or projection data can be recorded in a number of different projection directions with a digital x-ray detector. From these individual digital images recorded from different projection angles, i.e. from the image data belonging to these individual images, a three-dimensional image data set, for example composed of a number of layer images, that each represent one layer of the breast oriented in parallel to the reception surface of the x-ray detector, can be generated by image reconstruction methods. Such an image data set obtained by reconstruction is referred to below as a tomosynthetic 3D x-ray image. This technique enables tissue structures at a lower depth in the direction of propagation of the x-ray beam to be better detected.
Because of the incomplete scanning, i.e. the projections are only available from a restricted angular range, a reconstruction of a 3D x-ray image is only possible to a limited extent, so that the image quality of a tomosynthetic 3D x-ray image does not achieve the image quality known from computed tomography (CT). Thus, for example, the resolution in the direction of the central beam, referred to as the depth resolution, is reduced compared to the resolution in the layers at right angles to this. Also because of the incomplete scanning no quantitative values of the attenuation coefficient μ can be reconstructed, so that the character or impression of the image obtained with tomosynthesis also differs from the character of an image obtained with computer tomography methods. The imaging task on such cases focuses more in obtaining the best possible three-dimensional visualization of the object under the given projection conditions than on a quantitative reconstruction of the local absorption coefficient μ.
In tomography as well as in tomosynthesis the diagnostic evaluation capability of a reconstructed 3D x-ray image is heavily dependent of the reconstruction algorithms used, which in addition must be optimized with respect to the respective diagnostic problem.
The reconstruction methods known in the prior art for tomosynthesis are for example summarized in the aforementioned article by Dobbins et al. Essentially the methods employ unfiltered back projection (Niklason L T, Christian B T, Niklason L E, et al., “Digital Tomosynthesis in Breast Imaging”, Radiology 205, 399-406, 1997), non-linear back projection (Suryanarayanan S, Karellas A, Vedantham S, et al., “Evaluation of Linear and Nonlinear Tomosynthetic Reconstruction Methods in Digital Mammography”, Academic Radiology 8, 219-224, 2001), matrix inversion methods (D. J. Godfrey, A. Rader and J. T. Dobbins, III, “Practical Strategies for the Clinical Implementation of Matrix Inversion Tomosynthesis (MITS), Medical Imaging 2003: Physics of Medical Imaging, Proc. SPIE Vol. 5030 (2003), pp. 379-389), iterative (algebraic) methods (Wu T, Stewart A, Stanton M, et al., “Tomographic Mammography Using a Limited Number of Low-dose Cone-beam Projection Images”, Medical Physics 30, 365-380, 2003; Wu T, Moore R, Rafferty E A, Kopans D B, “A Comparison of Reconstruction Algorithms for Breast Tomosynthesis”, Medical Physics 31, 2636-2647, 2004), and filtered back projection (FBP) (Wu T, Moore R. Rafferty E A, Kopans D B, “A Comparison of Reconstruction Algorithms for Breast Tomosynthesis”, Medical Physics 31, 2636-2647, 2004; Lauritsch G, Haerer W H, “A Theoretical Framework for Filtered Backprojection in Tomosynthesis”, Proc. SPIE, 3338, 1127-1137, 1998 and U.S. Pat. No. 6,442,288).
With filtered back projection the measurement data provided by the x-ray detectors is filtered and subsequently projected back onto a volume matrix—the digital three-dimensional image of a part volume of the object. It is one of the most promising reconstruction methods since it is based on an analytical algorithm that can be obtained from the scanning geometry and is numerically very efficient and stable. Previously such methods have essentially used filters which are similar to the filters used in tomographic reconstruction. Thus, in the aforementioned article by Wu et al, for example, a filtered back projection specifically developed by Feldkamp for tomography on orbital paths using a cone-shaped x-ray beam bundle and known as the Feldkamp algorithm Feldkamp et al is applied.
Because of the basically incomplete nature of a tomosynthetic reconstruction in mammography, reconstruction algorithms, as are known from tomography, cannot simply be used for tomosynthetic reconstruction without any problems.